As used herein, the term inverter refers to a power converter that converts a direct current (“DC”) into an alternating current (“AC”). One application of an inverter is conversion of power from a DC source, such as, e.g., a battery, a photovoltaic cell or a fuel cell, for delivery to an AC utility grid. Certain inverters (e.g., those used by residential customers or small businesses) convert the DC power delivered by the DC source into single-phase AC power and deliver a sinusoidal current to the AC grid at the utility grid frequency. Inverters typically comprise switches that operate at a switching frequency that is high relative to the grid frequency. In an ideal inverter, the inverter output will be a pure single-frequency sinusoid at the utility grid frequency; in practice the inverter output will comprise switching artifacts, such as waveform ripple (e.g., deviation of the output current waveform from an ideal sinusoidal waveform), and will comprise noise components and frequency components at harmonics of the switching frequency. It is desirable that an inverter operate at high conversion efficiency while minimizing switching artifacts. Grid-connected inverters may, e.g., be required to meet the requirements of FCC Part 15, Classes A and B.
One way to reduce switching artifacts may be to use a passive filter at the output of an inverter. The size of the filter components selected may depend on the switching frequency: higher switching frequencies may result in both smaller filter components and less waveform ripple. However, higher switching frequencies may also result in lower conversion efficiency. Another way to reduce switching artifacts is to use a “multi-level inverter” in which series combinations of switches operate from multiple voltage sources to yield many possible switched output voltage levels. Both voltage-fed and current-fed multi-level converters are known.
In some energy systems, an inverter delivers power from one or more photovoltaic (“PV”) cells to an AC utility grid. Deriving power from PV cells and delivering it the AC grid presents a number of challenges, including operating cells at their maximum power points (“MPP”); achieving a high “utilization ratio” (i.e., the fraction of the total available power that is actually extracted); and minimizing ripple reflected back into the PV cells by the inverter. For practical reasons, PV cells are typically configured into PV panels that include series-parallel combinations of cells. A photovoltaic panel may, for example, includes 72 individual photovoltaic cells arranged to provide, e.g., 36V at 240 Watts. Panels and inverters may be further combined in series-parallel arrangements to scale up the total delivered voltage and power.
Panels and arrays of panels present additional challenges. For example, shading of even a few cells on a panel may result in substantial degradation of the total power delivered by the panel. In one topology, a “string-based” PV inverter system may comprise a single inverter that receives power from an array of several PV panels; in another “distributed” topology, several inverters may be provided, each inverter being configured to receive power from one panel. A properly designed distributed topology may exhibit improved utilization ratio and better overall MPP tracking than a string-based system.